Traditional propeller and liquid jet propulsion means accelerate a small portion of fluid to relatively high velocity because they transfer mechanical and fluid forces through the hull, and it is advantageous to minimize the size of hull penetrations to ease sealing. The relatively high energy density of these propulsion means favors turbulence that wastes power and creates noise. Additional complexities are required to direct the thrust of traditional propulsion means, usually requiring rudders, steering vanes, rotating fluid unions and the like. High energy density necessitates strong and therefore heavy power transmission components that exacerbate payload and buoyancy problems.
Most propulsion power sources are thermal devices that have higher efficiency when energy density is higher, but have small, highly stressed, and rapidly accelerated mechanical components. Thermal device efficiency generally degrades with delocalization because efficiency is closely related to temperature difference. Accelerating components are inherently acoustically noisy. Conventional hulls have a surface acoustic impedance sufficiently different from that of water that they are excellent sound reflectors. The high energy density renders conventional propellers acoustically noisy, particularly in shallow water. In addition, most propellers are single point catastrophic failure sites. Secondary outer hulls have been used to enhance vessel puncture and explosives protection. The weight, and the increased drag of bulklet outer hulls are penalties of puncture protection because the outer hull contributes nothing to the propulsion and guidance functions.
In contrast to traditional marine propulsion schemes, most aquatic animals have delocalized propulsion with low energy density. Aquatic mammals achieve very rapid acceleration by obviating much of the friction on their wetted surfaces, through compliance with the fluid, and by adding other fluids to the boundary layer. Eels and needle fish use traveling wave fins, while the variety of protozoan locomotion mechanisms seems without bound. Protozoa typically use traveling waves of the outer cell membrane, some with and some without augmenting cilia. Usually the entire external, and in some cases, internal surfaces are covered with propulsion and steering means. Thrust derived from moving body structures is easily directed by sending several different appropriate messages. Messages received by the body structures then elicit other messages, completing the propulsion act with little further effort by the animal. Efficient propulsion methods evolved in animals because they depended entirely on relatively slow metabolism. Their propulsion is quiet because noise wastes energy, and wasted energy eventually vitiates a species.
The following describes piezocellular propulsion that in part emulates natural processes.